The present invention relates to measuring the flow of a material through a flow line in which the bore is substantially uniform in cross-section except that one portion is of lesser cross-section and so acts as a flow restriction. As is well known, such restriction causes different pressures to exist in the material, whereby the flow rate of the material can be inferred from measurement of the pressure difference.
A variety of flow measurement techniques are known in the prior art, including the use of nozzles, venturis, orifices, vortexes, wedge-type restrictors and magnetic techniques. These techniques have associated advantages and disadvantages. Except for the magnetic flow meter, all of these techniques operate by producing a differential pressure which is a function of the flow rate. Among the differential pressure measuring elements, the wedge element retains a constant flow coefficient, Kd.sup.2 over a much larger range of pipe Reynolds number, particularly at low Reynolds numbers, relative to the other types of elements. This is an important advantage to the wedge-type element, in that it gives a reliable measure of flow rate to lower flows than the other restrictor-type elements.
U.S. Pat. No. 4,237,739, "Integral Flow Metering Assembly Using a Segmental Wedge", describes a commercially successful wedge-type flow measuring device that is installed in a pipe run. A one-piece rigid body has a cylindrical bore with a segmental wedge projecting therein. A differential pressure transmitter or the like, is mounted to sense the pressure developed on either side of the wedge in response to flow.
As fluid flows through the wedge restriction, the fluid velocity increases, resulting in a differential pressure across the pressure taps. The magnitude of the differential pressure is proportional to the volume flow rate by the square root law. At zero flow the differential is zero. As flow increases, the differential increases by the square of the flow rate. This relationship between flow and differential pressure prevails over a wide range of pipe Reynolds numbers, but deteriorates somewhat at low Reynolds numbers. Typically, with the wedge meter of the type shown in the +739 patent, the flow coefficient, Kd.sup.2, begins to deviate by more than 5% from theoretical, at Reynolds numbers under about 500.